Temperature detection of a magneto-optic recording medium for controlling irradiation of an erasure region

ABSTRACT

A method for reproducing a magneto-optical recording medium the recording pits of which are erased or relieved with rise in temperature of the recording medium caused by radiation of a readout beam, or an optical recording medium the reflectance of which is changed with rise in temperature of the recording medium caused by radiation of the readout beam, in which changes in the size of an effective reproducing region due to the temperature of the magneto-optical or optical recording medium may be inhibited. To this end, a temperature sensor 20 for sensing the temperature of the magneto-optical disc 11, for example, is provided and the laser power or the external magnetic field is controlled depending on the detected temperature by the temperature sensor 20 to maintain a constant size of the effective reproducing region at all times.

TECHNICAL FIELD

This invention relates to a method for reproducing signals from anoptical recording medium in which signals are read while a light beam isradiated on the recording medium. More particularly, it relates to amethod for reproducing signals from the recording medium capable ofreproducing the information recorded with high recording density.

BACKGROUND ART

An optical recording medium may be roughly classified into a read-onlymedium, such as a so-called compact disc, and a medium on which signalscan be recorded, such as a magneto-optical disc. With any of theseoptical recording media, it is desired to improve the recording densityto a higher level. It is because a data volume several to more than tentimes that of digital audio signals is required when recording digitalvideo signals and because a demand is raised for reducing the size ofthe recording medium, such as a disc and hence the size of a product,such as a player, even when recording digital audio signals.

Meanwhile, the recording density of recording the information on therecording medium is governed by the S/N ratio of the playback signals.In the typical conventional optical recording and reproduction, thetotal area of a beam spot SP, which is a radiation region of the readoutbeam, such as the laser beam for the optical recording medium, as shownin FIG. 1, is a playback signal region. Thus the reproducible recordingdensity is governed by the diameter D_(SP) of the beam spot of thereadout beam.

If, for example, the diameter D_(SP) of the beam spot SP of the readoutlaser beam is lesser than a pitch q of a recording pit RP, two recordingpits cannot be present in the spot SP, and the playback output waveformis as shown at B in FIG. 1, so that the playback signals can be read.However, if the recording pits SP are formed at a higher density, andthe diameter D_(SP) of the beam spot SP becomes larger than the pitch qof the recording pit RP, as shown at C in FIG. 1, two or more pits maybe present simultaneously in the spot SP, so that the playback outputwaveform becomes substantially constant as shown at D in FIG. 1. In thiscase, these two recording pits cannot be reproduced separately, so thatreproduction becomes infeasible.

The spot diameter D_(SP) depends on the wavelength λ of the laser beamand on the numerical aperture NA. It is this spot diameter D_(SP) thatgoverns the pit density along the scanning direction of the read-outbeam or the recording track direction, or the so-called line density,and the track density conforming to the track interval betweenneighboring tracks in a direction at right angles to the scanningdirection of the readout beam, or the so-called track pitch. Theopto-physical limits of the line density and the track density are setby the wavelength λ of the readout beam source and the numericalaperture NA of the objective lens and the read-out limit of 2NA/λ isgenerally accepted as long as the spatial frequency at the time ofsignal reproduction is concerned. For this reason, for achieving highdensity of the optical recording medium, it is necessary to diminish thewavelength λ of the light source of the reproducing optical system, suchas a semiconductor laser, as well as to enlarge the numerical apertureNA of the objective lens.

The present Applicant has already proposed an optical recording mediumin which the recordable line recording density as well as the trackdensity may be increased without changing the spot diameter of thereadout beam spot, and a method for reproducing the optical recordingmedium. The optical recording medium capable of reproducing the highdensity information in this manner may be enumerated by amagneto-optical recording medium capable of recording informationsignals and a variable reflectance type optical recording medium atleast capable of reproducing information signals.

The above-mentioned magneto-optical recording medium includes a magneticlayer, such as a rare earth-transition metal alloy thin film, depositedon a major surface of a transparent substrate or light-transmittingsubstrate of e.g. polycarbonate, together with a dielectric layer and asurface protecting layer. The magnetic layer has an easy axis ofmagnetization perpendicular to the film surface and exhibits superiormagneto-optical effect. The laser beam is irradiated from the side ofthe transparent substrate for recording/reproducing information signals.Signals are recorded on the magneto-optical recording medium byso-called thermo-magnetic recording in which the magnetic layer islocally heated by e.g. laser beam radiation to close to the Curietemperature to reduce the coercivity to zero in this region and arecording magnetic field is applied to this region from outside formagnetization in the direction of the recording magnetic field. Therecorded signals may be reproduced by taking advantage of themagneto-optical effect, such as the so-called magnetic Kerr effect orFaraday effect, in which the plane of polarization of the linearlypolarized light, such as laser beam, is rotated in the direction of themagnetization of the magnetic layer.

The variable reflectance type optical recording medium is produced bydepositing a material changed in reflectance with temperature on atransparent substrate on which phase pits are formed. During signalreproduction, the readout beam is radiated on the recording medium andthe reflectance is partially changed within the scanning spot of thereadout beam to read out the phase pits.

In connection with the above-mentioned magneto-optical recording medium,high density reproduction or so-called high resolution reproduction ishereinafter explained.

The present Applicant has previously proposed in e.g. Japanese PatentLaid-Open Publication No. 1-143041 (1989) and Japanese Patent Laid-OpenPublication No. 1-143042 (1989) a method for reproducing informationsignals for a magneto-optical recording medium wherein information bits(magnetic domains) are enlarged, diminished or reduced to zero forimproving the playback resolution. The essential point of the technologyconsists in that the recording magnetic recording layer is anexchange-coupled multilayer film composed of a reproducing layer, anintermediate layer and a recording layer, and in that the magneticdomain heated by the playback light beam during reproduction isenlarged, diminished or erased at a zone of higher temperatures fordiminishing the inter-bit interference during reproduction to render itpossible to reproduce signals of a period lower than the lightdiffraction threshold. There is also proposed in the applicationdocuments of JP Patent Application No. 1-229395 (1989) a technology inwhich the recording layer of the magneto-optical recording medium isformed by a multilayer film including a magnetically coupled reproducinglayer and a recording holding layer, the direction of magnetization isaligned in advance to an erased state, the reproducing layer is heatedto a temperature higher than a predetermined temperature by irradiationof the laser light and in which magnetic signals written on therecording holding layer only in this heated state are read out whilebeing transcribed on the reproducing layer to eliminate signal crosstalkto improve the line recording density and the track density.

The above-described high density reproducing technology may be roughlyclassified into an erasure type and a relief type, shown schematicallyin FIGS. 2 and 3, respectively.

Referring first to FIGS. 2A, 2B and 2C, the erasure type high densityreproduction technique is explained. With the erasure type, therecording medium, on which information recording pits RP are exhibitedat room temperature, is heated by irradiation of a laser light LB toproduce an erased region ER within the beam spot SP of the radiatedlaser beam LB, as shown at B in FIG. 2, and the recording pit RP withina remaining region RD within the beam spot SP is read, by way ofachieving reproduction with improved line density. In sum, thistechnique consists in that, when reading the recorded pit RP within thebeam spot SP, the erased region ER is used as a mask to narrow the widthd of the read-out region (playback region) RD to provide forreproduction with an increased density along the scanning direction ofthe laser light (track direction), that is the so-called line recordingdensity.

The recording medium for erasure type high density reproduction has anexchange-coupled magnetic multilayer film structure composed of anamorphous rare earth for photomagnetic recording (Gd, Tb)-iron group(Fe, Co) ferrimagnetic film. In an example shown at A in FIG. 2, therecording medium has a structure in which a reproducing layer as a firstmagnetic film 61, an interrupting layer (intermediate layer) as a secondmagnetic layer 62 and a recording holding layer as a third magneticlayer 63, deposited in this order on a major surface (the lower surfacein the drawing) of a transparent substrate 60 formed of e.g.polycarbonate. The first magnetic layer (reproducing layer) 61 is e.g. aGdFeCo layer with a Curie temperature T_(c1) >400° C., while the secondmagnetic layer (interrupting layer or an intermediate layer) 62 is e.g.a TbFeCoAl film having a Curie temperature T_(c2) of 120° C. and thethird magnetic layer (recording holding layer) is e.g. a TbFeCo layerwith a Curie temperature T_(c3) of 300° C. Meanwhile, arrow marks in themagnetic films 61 to 63 shown at C in FIG. 2 represent the direction ofmagnetization of the magnetic domains. H_(read) represents the directionof the reproducing magnetic domain.

The reproducing operation is briefly explained. At an ambienttemperature below a predetermined temperature T_(OP), the layers 63, 62and 61 of the recording medium are magnetically coupled in the state ofstatic magnetic coupling or exchange coupling, while the recordingmagnetic domain of the recording holding layer 63 is transcribed to thereproducing layer 61 by means of the interrupting layer 62. If the laserbeam LB is radiated on the recording medium for raising the mediumtemperature, changes in the medium temperature are produced with a timedelay with the scanning of the laser beam, so that a region at atemperature higher than the predetermined temperature T_(OP), that isthe erased region ER, is shifted slightly towards the rear side of thelaser spot SP in the laser scanning direction. At the temperature higherthan the predetermined temperature T_(OP), the magnetic coupling betweenthe recording holding layer 63 and the reproducing layer 61 disappearsand the magnetic domains of the reproducing layer 61 are aligned in thedirection of the reproducing magnetic field H_(read), with the recordingpits being erased on the medium surface. A region RD of the scanningspot SP, excluding a superposed region with the region ER where thetemperature is higher than the predetermined temperature T_(OP),substantially represents a reproducing region. That is, the laser spotSP of the laser beam is partially masked by the region ER where thetemperature becomes higher than the predetermined temperature T_(OP), sothat the small unmasked region becomes the reproducing domain RD toachieve high density reproduction.

Since pits may be reproduced by detecting e.g. the Kerr rotation angleof the beam reflected from a small reproducing region (readout regionRD) in which the scanning spot SP of the laser beam is not masked by themasking region (erased region ER), the beam spot SP is equivalentlyincreased in diameter for increasing the line recording density and thetrack density.

In the relief type high density reproducing technique, shown at B inFIG. 3, the recording medium in a state in which information recordingpits RP are erased at ambient temperature (initialized state) isirradiated with a laser beam and thereby heated to form a signaldetecting region DT, as a recording relieved region, within the beamspot SP of the laser beam, and only the recording pit RP within thissignal detecting region DT is read for improving the playback linedensity.

The recording medium for such high density relief reproduction has amagnetic multilayer structure according to magnetostatic coupling ormagnetic exchange coupling. In an example shown at A in FIG. 3, areproducing layer 71 as a first magnetic layer, a reproduction assistantlayer 72 as a second magnetic layer, an intermediate layer 73 as a thirdmagnetic layer 73 and a recording holding layer 74 as a fourth magneticlayer are stacked sequentially on a major surface (the lower surface inFIG. 3) of a transparent substrate 70 formed of e.g. polycarbonate. Thefirst magnetic layer (reproducing layer) 71 is formed e.g. of GdFeCo andhas a Curie temperature T_(c1) >300° C., the second magnetic layer(reproduction assistant layer) 72 is formed e.g. of TbFeCoAl and has aCurie temperature T_(c2) ≈120° C., the third magnetic layer(intermediate layer) 73 is formed e.g. of GdFeCo and has a Curietemperature T_(c3) ≈250° C. and the fourth magnetic layer (recordingholding layer) 74 is formed e.g. of TbFeCo and has a Curie temperatureT_(c4) ≈250°. The magnitude of an initializing magnetic field H_(in) isselected to be larger than a magnetic field H_(cp) inverting themagnetization of the reproducing layer (H_(in) >H_(cp)) and sufficientlysmaller than the magnetizing field H_(cr) inverting the magnetization ofthe recording holding layer (H_(in) <<H_(cp)). The arrows in themagnetic layers 71, 72 and 73 at C in FIG. 3 indicate the direction ofmagnetization in each domain, H_(in) indicates the direction of theinitializing magnetic field and H_(read) the direction of thereproducing magnetic field.

The recording holding layer 74 is a layer holding recording pits withoutbeing affected by the initializing magnetic field H_(in), thereproducing magnetic field H_(read) or the reproducing temperature, andexhibits sufficient coercivity at room temperature and at the playbacktemperature.

The intermediate layer 73 exhibits perpendicular anisotropy less thanthat of the reproduction assistant layer 72 or the recording holdinglayer 74. Therefore, a magnetic wall may exist stably at theintermediate layer 73 when the magnetic wall is built between thereproducing layer 71 and the recording layer 74. For this reason, thereproducing layer 71 and the reproduction assistant layer 72 maintainthe erased state (initialized state) in stability.

The reproduction assistant layer 72 plays the role of increasingcoercivity of the reproducing layer 71 at room temperature, so thatmagnetization of the reproducing layer 71 and the reproduction assistantlayer 72 may exist stably despite presence of the magnetic wall. On theother hand, coercivity is decreased acutely during reproduction in thevicinity of the reproduction temperature T_(s) so that the magnetic wallconfined in the intermediate wall 73 is expanded to the reproductionassistant layer 72 to invert the reproducing layer 71 ultimately toextinguish the magnetic wall. By this process, pits are caused to appearin the reproducing layer 71.

The reproducing layer 71 has a low inverting magnetic field H_(cp) sothat the domains of overall surface of the layer 71 may be aligned bythe initializing field H_(in). The aligned domains are supported by thereproduction assistant layer 72 and may thereby be maintained stablyeven if there exist a magnetic field between the layer and thereproduction assistant layer 74. Recording pits are produced by thedisappearance of the magnetic wall between the layer and the recordingholding layer 74 during reproduction, as described above.

The operation during reproduction is explained briefly. The domains ofthe reproducing layer 71 and the reproduction assistant layer 72 arealigned before reproduction in one direction (in an upward direction inFIG. 3) by the initializing magnetic field H_(in). At this time, amagnetic wall (indicated in FIG. 3 by a transversely directed arrow) ispresent stably so that the reproducing layer 71 and the reproductionassistant layer 72 are stably maintained in the initialized state.

A reproducing magnetic field H_(read) is applied in an inverse directionwhile a laser beam LB is radiated. The reproducing magnetic fieldH_(read) needs to be in excess of the magnetic field which inverts thedomains of the reproducing layer 71 and the reproduction assistant layer72 at a reproduction temperature T_(RP) following temperature increaseby laser irradiation to cause extinction of the magnetic field of theintermediate layer 73. The reproducing magnetic field also needs to beof a such a magnitude as not to invert the direction of magnetization ofthe reproducing layer 71 and the reproduction assistant layer 72.

With scanning by the laser beam, temperature changes in the medium areproduced with a delay, so that the region whose temperature exceeds apredetermined reproducing temperature T_(RP) (recording relieved region)is shifted slightly from the beam spot SP towards the rear side alongthe scanning direction. With the temperature above the predeterminedreproducing temperature T_(RP), coercivity of the reproduction assistantlayer 72 is lowered, so that, when the reproducing magnetic fieldH_(read) is applied, the magnetic wall is caused to disappear so thatthe information of the recording holding layer 74 is transcribed on thereproducing layer 71. Thus a region within the beam spot SP which doesnot reach the reproducing temperature T_(RP) is masked and the remainingregion within the beam spot SP becomes the signal detecting region(reproducing region) DT which is the recording relieved region. Highdensity reproduction may be achieved by detecting e.g. the Kerr rotationangle of a plane of polarization of the reflected beam from the signaldetecting region DT.

That is, the region within the beam spot SP of the laser beam LB whichdoes yet not reach the reproduction temperature T_(RP) is a mask regionin which recording pits are not displayed, while the remaining signaldetecting region (reproducing region) DT is smaller in area than thespot diameter, so that the line recording density and the track densitymay be increased in the same manner as described above.

There is also devised a high density reproducing technique consisting ina combination of the erasure type and the relief type. With thistechnique, the laser beam is radiated to the recording medium in aninitialized state thereof in which recording pits are extinct at roomtemperature. This heats the recording medium for forming a recordingrelieved region at a site slightly deviated towards the rear side of thebeam spot of the radiating laser beam, simultaneously forming an erasedregion of a higher temperature within the recording relieved region.

In the specification and the drawings of our co-pending JP PatentApplication No. H 3-418110 (1991), there is proposed a signalreproducing method for a magneto-optical recording medium wherein amagneto-optical recording medium having at least a reproducing layer, anintermediate layer and a recording holding layer is employed, a laserbeam is radiated and a reproducing magnetic field is applied to thereproducing layer, a temperature distribution generated by the laserradiation is utilized to produce a region where an initialized state ismaintained, a region to which the information of the recording holdinglayer is transcribed and a region the domains of which are aligned inthe direction of the reproducing magnetic field, in a field of view ofthe lens, to produce a state equivalent to optically masking the fieldof view of the lens to increase the line recording density and the trackdensity as well as to assure satisfactory frequency characteristics atthe time of reproduction, there being no risk that, even if thereproducing power is fluctuated, the region of transcription of theinformation of the recording holding layer be diminished or enlarged.

According to the above-described high density reproducing techniqueemploying such magneto-optical recording medium, only the read regionRD, which is in effect the signal reproducing region, or the recordingpit RP within the signal detecting region DT, is read within the beamspot SP. Since the size of the read region RD or the signal detectionregion DT is smaller than the size of the beam spot SP, the distancebetween adjacent pits in the directions along and at right angles to thelaser beam scanning direction may be reduced to raise the line densityand the track density to increase the recording capacity of therecording medium.

Meanwhile, with the above-described method for reproducing thehigh-density information, even although the external reproducingmagnetic field is constant and the laser light power is constant, thesize of the region RD of FIG. 2 or that of the region DT of FIG. 3, asthe reproducing region, is fluctuated with changes in the temperature ofthe recording medium, such as the magneto-optical disc, brought about bychanges in ambient temperature.

For example, with the erasure type reproducing method, explained inconnection with FIG. 2, if the temperature of the recording medium, suchas the magneto-optical disc, is high, the state of temperaturedistribution shows a shift towards a higher temperature, as shown by acurve a at B in FIG. 4, so that the erased region exceeding the Curietemperature T_(c) (mask region) is as shown at ER_(HT) at A in FIG. 4,so that the effective reading region (reproducing region) RD isdiminished.

On the other hand, if the temperature of the recording medium is low,the state of temperature distribution shows a shift towards a lowertemperature, as shown by a curve b at B in FIG. 4, so that the erasedregion exceeding the Curie temperature T_(c) (mask region) is as shownat ER_(LT) at A in FIG. 4, so that the effective reading region(reproducing region) RD is diminished.

With the relief type, as will become apparent from its operatingprinciple, if the temperature of the magneto-optical recording medium ishigh, the reproducing region is increased, whereas, if the temperatureof the magneto-optical recording medium is low, the reproducing regionis diminished.

As described above, if, with the erasure type reproducing method or withthe relief type reproducing method, the effective reproducing region isfluctuated, there is a risk that stable reproduction with a high S/Nratio cannot be achieved.

The same may be said when reproducing a variable reflectance opticalrecording medium by way of high density reproduction or ultra highresolution reproduction. That is, since the portion within the readoutbeam which is changed in reflectance is changed in size with changes inthe medium temperature, the high reflectance portion, which is in effectthe reproducing region, is fluctuated in size with the mediumtemperature, so that stable reproduction can occasionally not beachieved.

In view of the above-described status of the art, it is an object of thepresent invention to provide a method for reproducing an opticalrecording medium in which, even although the temperature of themagneto-optical recording medium of the variable reflectance typeoptical recording medium is changed, the size of the effectivereproducing region may be maintained constant to assure stable readingof information signals.

DISCLOSURE OF THE INVENTION

According to the present invention, there is provided a method forreproducing an optical recording medium comprising a recording layer anda reproducing layer, the recording and reproducing layers beingmagnetically coupled to each other in steady state, the methodcomprising extinguishing magnetic coupling between the recording layerand the reproducing layer in a region the temperature of which is raisedto a temperature higher than a predetermined temperature by irradiationof a readout laser beam during reproduction, and reading the recordinginformation held by said recording layer in an area of an irradiatedregion other than the magnetic coupling extinguished region,characterized by detecting the temperature of said optical recordingmedium and controlling the size of the magnetic coupling extinguishedregion based on the detected temperature.

It is preferred to control the intensity of the readout beam based onthe detected temperature of the recording medium. It is also preferredto control the size of the first region based on the level of the signalread out from the reproducing layer.

According to the present invention, there ia also provided a method forreproducing an optical recording medium comprising a recording layer anda reproducing layer, the method comprising aligning the domains of saidreproducing layer, transcribing the recording information held by therecording layer in a region of the recording medium in which thetemperature is raised to a temperature higher than a predeterminedtemperature by irradiation of a readout beam during reproduction forrelieving the transcribed recording information, and reading out therecording information from a relieved region of said reproducing layer,characterized by detecting the temperature of the optical recordingmedium, and controlling the size of the relieved region based on thedetected temperature.

It is preferred to impress a reproducing magnetic field whentranscribing and relieving the recording information held by therecording layer during reproduction to the reproducing layer, and tocontrol the intensity of the reproducing magnetic field based on thedetected temperature of the recording medium. It is also preferred tocontrol the size of the relieved region based on the level of a signalread from said reproducing layer.

According to the present invention, there is additionally provided amethod for reproducing an optical recording medium having phase pitsformed thereon in accordance with signals and having its reflectancechanged with temperature, the method comprising radiating a readout beamto the recording medium and reading out the phase pits while partiallychanging the reflectance within a scanning spot of a readout beam,characterized by detecting the temperature of the optical recordingmedium and controlling the size of a portion within the scanning spot ofthe readout light beam in which reflectance is changed depending on thedetected temperature.

It is preferred to control the intensity of the readout light beam basedon the detected temperature of the recording medium and to control thesize of said portion in which reflectance is changed depending on thelevel of the signal read out from the optical recording medium.

In this manner, according to the present invention, the size of theregion where the magnetic coupling has been extinguished, the relievedregion or the portion where the reflectance is changed may be controlledeven although the temperature of the optical recording medium ischanged, so that the size of the effective reproducing region may berendered constant to achieve reproduction with high S/N ratio as well asto realize high density reproduction or reproduction with ultra-highresolution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1D are a view for illustrating the relation between the spotdiameter of a laser beam and the recording density of reproduciblerecording pits.

FIGS. 2A-2C are a view for illustrating an erasure type magneto-opticalrecording medium, a method for reproducing the recording medium and aneffective reproducing region of the recording medium.

FIGS. 3A-3C are a view for illustrating a relief type magneto-opticalrecording medium, a method for reproducing the recording medium and aneffective reproducing region of the recording medium.

FIGS. 4A and 4B are a view for illustrating that the effectivereproducing region is changed with changes in the temperature of themagneto-optical recording medium.

FIG. 5 is a block diagram showing essential parts of a disc reproducingapparatus to which an embodiment of the reproducing method for themagneto-optical recording medium according to the present invention isapplied.

FIG. 6 is a view for illustrating that a mask region is changed bychanging a laser power.

FIG. 7 is a view for illustrating that a mask region is changed bychanging an external magnetic field.

FIG. 8 is a block diagram showing essential parts of a disc reproducingapparatus to which another embodiment of the reproducing method for themagneto-optical recording medium according to the present invention isapplied.

FIG. 9 is a block diagram showing essential parts of a disc reproducingapparatus to which a still another embodiment of the reproducing methodfor the magneto-optical recording medium according to the presentinvention is applied.

FIG. 10 is a schematic cross-sectional view showing essential parts ofan example of a phase change type optical disc as typical of a variablereflectance optical disc employed in the embodiment shown in FIG. 9.

FIG. 11 is a schematic cross-sectional view showing another example ofthe phase change type optical disc.

FIG. 12 is a schematic cross-sectional view showing still anotherexample of the phase change type optical disc.

FIG. 13 is a view showing a phase change state for explanation of theabove-mentioned phase change type optical disc.

FIG. 14 is a view showing another phase change state for explanation ofthe above-mentioned phase change type optical disc.

FIG. 15A and 15B are views showing the relation between the temperaturedistribution and a readout light spot for explanation of theabove-mentioned phase change type optical disc.

FIG. 16 is a schematic cross-sectional view showing essential parts ofanother example of the variable reflectance optical disc employed in theembodiment shown in FIG. 9.

FIG. 17 is a graph showing the state of change in reflectance spectralcharacteristics with changes in temperature in an interference filter.

BEST MODE OF CARRYING OUT THE INVENTION

Referring to the drawings, certain embodiments of an optical recordingmedium according to the present invention will be explained. First, anembodiment in which the present invention is applied to amagneto-optical recording medium as a recordable medium, and then anembodiment in which the present invention is applied to a variablereflectance optical recording medium as a recordable medium will beexplained.

In FIG. 5, a magneto-optical recording medium is a magneto-optical disc11, to which the above-mentioned erasure type or relieved typereproducing method is applied. The rotating driving system for themagneto-optical disc 11 may be of a constant angular velocity (CAV) typeor of a constant linear velocity type (CLV) type.

For example, the magneto-optical disc to which the erasure typereproducing method is applied has an exchange-coupled magneticmultilayer film structure composed of an amorphous rare earth (Gd,Tb)-iron group (Fe, Co) ferrimagnetic film for magneto-optical recordingwhich is made up of a recording holding layer formed e.g. of TbFeCo witha Curie temperature of 300° C., an interrupting layer (intermediatelayer) of e.g. TbFeCoAl with a Cure temperature T of 120° C. and areproducing layer of e.g. GdFeCo with a Curie temperature of not lowerthan 400° C. The magneto-optical disc to which the relief typereproducing method is applied is such a disc in which the recordingholding layer is formed of e.g. TbFeCo with a Curie temperature of 250°C., the intermediate layer is formed e.g. of GdFeCo with a Curietemperature of 250° C., the reproduction assistant layer is formed e.g.of TbFeCoAl with a Curie temperature of 120° C. and the reproducinglayer is formed e.g. of GdFeCo with a Curie temperature of not lowerthan 300° C.

As a readout beam, a laser light beam from a laser source, such as asemiconductor laser, is incident on the reproducing layer of themagneto-optical disc 11.

In the present embodiment, a reproducing magnetic field H_(read) isgenerated by the driving current supplied to a magnetic field generatingcoil 31 from a driver 32. The magnetic field generating coil 31 isarranged facing the laser source 11 on the opposite side of themagneto-optical disc 11 with respect to the laser beam side. A referencevalue M_(ref) from a reference value generating circuit 23 is suppliedto the driver 22 and the magnitude of the reproducing magnetic fieldH_(read) generated by the coil 21 is set so as to be a constant valueconforming to this reference value.

In accordance with he above-mentioned erasure or relief type reproducingmethod, the reflected light from the reproducing region RD or DT withinthe beam spot of the laser beam is incident on the reproducingphotodetector 13 by optical means, not shown, for photoelectricconversion.

Output signals of the photodetector 13 are supplied via a head amplifier14 to a signal processing circuit 15 to produce RF signals which aresupplied to a data reproducing system for demodulation.

Part of the laser beam from the laser source 12 is incident on a laserpower monitoring photodetector 16. The photoelectrically convertedoutput of the photodetector 16 is supplied to an automatic powercontroller 17, in which the output of the photodetector 16 and areproducing laser power setting reference value REF from a latch circuit22 are compared to each other. Outputs of the comparison error aresupplied to a laser driving circuit 18 so that the output power from thelaser source 12 is controlled so as to conform to the reproducing laserpower setting reference value REF.

In the present embodiment, the reproducing laser power setting referencevalue REF is adapted to conform to the temperature of themagneto-optical disc 11, as will now be explained.

That is, a temperature sensor 20, such as a thermistor, is provided inthe vicinity of the magneto-optical disc 11 for sensing the temperatureof the magneto-optical disc 11. A temperature output sensed by thetemperature sensor 20 is supplied to a ROM 21, which stores a table ofthe reproducing laser power setting reference value REF, as a readoutaddress therefor. Different reproducing power setting reference valuesREF are read out from ROM 21 depending on temperature outputs fromtemperature sensor 20. The read-out setting reference value REF islatched by latch circuit 22 by timing signals from a timing signalgenerator 23 before being supplied to automatic power controller 16. Asa result, the output power of laser light source 12 is controlled so asto conform to the setting reference value conforming in turn to theprevailing temperature of the magneto-optical disc 11.

As explained previously, if the temperature of the magneto-optical disc11 is changed, the temperature distribution by the laser beam scanningspot is shifted with the disc temperature. However, if the laser outputpower is changed, the size of a region in excess of a predeterminedthreshold temperature T8 is changed, as shown at S1 and S2 in FIG. 6,even although the temperature of the magneto-optical disc 11 isconstant. Thus, by controlling the laser power as described above, thesize of the reproducing regions RD and DT may remain constant evenalthough the temperature of the magneto-optical disc 11 is changed.

ROM 21 stores a table for laser power setting reference value REFcorresponding in a one-for-one relationship to minute temperature rangesobtained by subdividing the temperature range of the magneto-opticaldisc 11 in accordance with the storage capacity of ROM 21. Thereproducing laser power setting reference values REF which will give anoptimum constant sizes of the effective reproducing region RD or DT forreproduction for these temperature ranges of the magneto-optical disc 11of the erasure or relief type are previously detected, and thereproducing laser power setting reference values REF associated withthese temperature ranges are stored in ROM 21.

Whether or not the size of the reproducing region RD or DT is of anoptimum constant value may be detected depending on whether or not theRF signal level from the signal processing circuit 15 is of apredetermined value when the predetermined reference pattern informationis reproduced.

Therefore, while the reproducing laser power setting reference value REFconforming to the temperature of the magneto-optical disc 11 detected bytemperature sensor 20 is perpetually read out from ROM 21, thereproducing laser power setting value REF need not be changedcontinually, but needs only to be changed at a time point whichsufficiently takes account of temperature changes of the magneto-opticaldisc 11. In the present embodiment, an output of ROM 21 is latched bylatch circuit 22 by timing signals from timing signal generator 23, andthe reproducing laser power setting reference value REF is changed atthe time points of the timing signals. These time points are the starttime point and each predetermined time interval in which temperaturechanges are though to e produced, for example, each ten minutes, forexample, and timing signal generator 23 generates timing signals at thereproducing start time point and each ten n minutes since thereproducing start time point.

Since the reproducing region RD or DT in the erasure type or relief typereproducing method may be maintained constant by controlling the laserpower, even although the temperature of the magneto-optical disc 11 ischanged with changes in ambient temperature, stable reproduction may beachieved at all times.

Meanwhile, an IR sensor for sensing the surface temperature of themagneto-optical disc 11 itself may be used as temperature sensor 20 inplace of the sensor for sensing the temperature of the magneto-opticaldisc 11 by detecting the ambient temperature of the magneto-opticaldisc, such as thermistor, for a more accurate controlling operation.

A circuit for finding the reproducing laser power setting referencevalue REF from the information of the detected temperature from thetemperature sensor may also be used as generator of the reproducinglaser power setting reference value REF in place of ROM 21.

Although the laser power is controlled in the above embodiment forrendering the size of the reproducing regions RD and DT constant despitechanges in the temperature of the magneto-optical disc, similar effectsmay also be achieved by controlling the external magnetic field(reproducing magnetic field H_(read)).

That is, with the erasure type reproducing method, for example, thetemperature at which the mask region (erased region) ER starts to begenerated is precisely not the Curie temperature T_(c2) of theintermediate layer 62, but is correlated with the reproducing magneticfield H_(read), and is a temperature such that

    H.sub.c1 +H.sub.w <H.sub.read                              (1)

wherein H_(ci) is coercivity of the reproducing layer 61 and H_(w) isthe exchange-coupling force between layers 61 and 63. Theexchange-coupling force H_(w) between the reproducing layer 61 and therecording layer 63 becomes lower with rise in temperature and becomesequal to zero at the Curie temperature T_(c2) of the intermediate layer62.

FIG. 7 shows temperature characteristics of H_(c1) +H_(w). In FIG. 7,T_(c1) is the Curie temperature of the reproducing layer 61. At atemperature higher than the Curie temperature T_(c2) of the intermediatelayer, coercivity is similar to that when only one reproducing layer isprovided.

For aligning the domains of the reproducing layer 61 of themagneto-optical disc, it suffices to apply a magnetic field larger thanH_(c1) +H_(w), as shown by the formula (1). Therefore, if H_(r0) isapplied as reproducing magnetic field H_(read) in FIG. 7 for the sametemperature distribution, the range having the temperature higher thanthe Curie temperature T_(c2) becomes the mask region ER. However, if thestrength of the reproducing magnetic field H_(read) is H_(r1), the rangeup to a temperature T_(a) lower than the Curie temperature T_(c2)becomes the mask region ER. In this manner, the size of the mask regionis changed with the strength of the reproducing magnetic field H_(read),so that the reproducing region RD is changed in size.

Therefore, by changing the external magnetic field, such as thereproducing magnetic field H_(read), depending on the temperature of themagneto-optical disc 11, the reproducing region may perpetually berendered constant.

The reproducing magnetic field may similarly by controlled in the caseof the relief type reproducing method for rendering the size of thereproducing region DT constant.

FIG. 8 shows an embodiment of essential parts of a reproducing apparatusin which the reproducing magnetic field is controlled depending on thetemperature of the magneto-optical disc.

In the present embodiment, a constant laser power setting referencevalue REF from a reference value generator 19 is supplied to anautomatic power controlling circuit 17 and an output laser power of thelaser light source 12 is controlled to constant value conforming to thisreference value.

A reference value M_(ref) from a reference value generator 33 issupplied to an adder 34 where it is added to a correction value from alatch circuit 35. A driving signal consisting in the sum value issupplied to a driver 32. Thus the strength of the reproducing magneticfield H_(read) is of a predetermined value conforming to the referencevalue REF, if the correcting value is zero, so that the strength ischanged around the predetermined value depending on the correctingvalue.

In the present embodiment, a ROM 36 is provided for storing a table ofthe correcting values associated with the temperature of themagneto-optical disc 11, and a detected temperature output from thetemperature sensor 20 is entered as a readout address for ROM 36. In thepresent embodiment, the correcting values stored in ROM 36 are of suchvalues that the sizes of the reproducing regions RD and DT becomeperpetually constant for the respective temperatures of themagneto-optical disc 11.

Whether or not the size of the reproducing region RD and Dt is constantmay again be detected depending on whether or not the RF signal levelfrom signal processor 15 is of a constant value when the information ofa predetermined reference pattern is reproduced.

The correcting values read from ROM 36 are latched by latch circuit 35by timing signals from timing signal generator 37. The correcting valueslatched by latch circuit 35 are supplied to an addition circuit 34 forcontrolling he strength of the reproducing magnetic field depending onthe temperature of the magneto-optical disc 11 for perpetually renderingthe size of the reproducing region RD or DT.

Meanwhile, a circuit for calculating the correcting values from theinformation concerning the detection temperature from the temperaturesensor may be used as the correcting value generator in place of ROM 36.

The laser power and the external magnetic field my also be controlledsimultaneously, in place of independently controlling the laser powerand the external magnetic field depending on the temperature of themagneto-optical disc, as in the above embodiment.

The present invention may also be applied to a magneto-optical disc ofthe type consisting in a mixture of the erasure and relief types.

With the high density reproducing technology employing thesemagneto-optical recording media, recording pits may be read only fromreproducing regions narrower than the beam spot area. Besides, theeffective size of the reproducing region may perpetually be renderedconstant despite changes in the temperature of the recording media toprovide for stable reproduction. The result is that high density may beachieved to increase the capacity of the recording medium to producehigh quality reproducing signals at all times.

The above embodiments are directed to a recordable magneto-opticalrecording medium. The following description is made of an embodiment inwhich the present invention is applied to a variable reflectancemagneto-optical recording medium.

As the technique concerning the variable reflectance optical recordingmedium, the present Applicant has already proposed a signal recordingmethod for an optical disc in the specification and drawings of JapanesePatent Application NO. H-2-94452 (1990), and an optical disc in thespecification and drawings of Japanese Patent Application No. H-2-291773(1990). In the former, a signal reproducing method for an optical discis disclosed, whereby a readout beam is radiated to an optical disc onwhich phase pits are formed depending on signals and which has thereflectance changed with temperatures, and the phase pits are read whilethe reflectance is partially changed within a scanning beam spot of thereadout beam. In the latter, an optical disc of a so-called phase changetype is disclosed, in which a layer of a material changed in reflectancewith phase changes is formed on a transparent substrate which hasreflectance changed with phase changes and in which, when the disc isirradiated with the readout beam, the layer partially undergoes phasechanges within the scanning spot of the readout beam and is reset afterreadout is terminated.

The material of the layer is preferably such a material in which a layerof a phase change material which may be crystallized after being meltedand in which, when the layer is irradiated with the readout beam, thematerial is changed into a liquid phase within the scanning spot of thereadout beam within the melted and crystallized region as to be changedin reflectance and be reset to a crystallized state after readout isterminated.

FIG. 9 shows essential parts of a disc reproducing apparatus to which isapplied a modified embodiment of the reproducing method of the presentinvention employing the variable reflectance type optical recordingmedium, above all, the phase change type optical disc.

In FIG. 9, an optical disc 100 is a variable reflectance type opticaldisc, above all, a phase change type optical disc. The disc in which thereflectance of a portion thereof irradiated with the readout laser beamand raised in temperature is lower than that of the remaining portioncorresponds to the erasure type magneto-optical disc, while the in whichthe reflectance of a portion thereof raised in temperature is lower thanthat of the remaining portion corresponds to the erasure typemagneto-optical disc. The present embodiment is applicable not only toboth types of the phase change type optical discs, but also to variablereflectance type optical discs based on other operating principles.

The arrangement shown in FIG. 9 is the same as that shown in FIG. 5except that the magnetic field generating coil 21, driver 22 and thereference value generator 33 are eliminated and a variable reflectancetype optical disc 100 is used in place of the magneto-optical disc 11.

That is, a light beam from the laser light source 12 is incident on theoptical disc 100 and the beam reflected from a reproducing region withina laser beam spot is incident on a reproducing photodetector 13 toundergo photoelectric conversion while output signals from photodetector13 are supplied by means of head amplifier 14 to a signal processor 15to produce RF signals which are supplied to a data reproducing systemfor demodulation.

Part of the laser from the laser light source 12 is incident on aphotodetector 16 for laser power monitoring to undergo photoelectricconversion before being supplied to an automatic controller 17. In theautomatic controller 17, an output of the photodetector 16 and thereproducing laser power setting reference value REF from latch circuit22 are compared to each other. A comparison error output from controller17 is supplied to a laser driving circuit 18 for controlling the outputpower of the laser light source 12 to a value conforming to thereproducing laser power setting reference value REF.

In the present embodiment, the reproducing laser power setting referencevalue REF is adapted for conforming to the temperature of the opticaldisc 100.

A temperature sensor 20, such as a thermistor, is provided in thevicinity of the magneto-optical disc 100 for sensing the temperature ofthe magneto-optical disc 100. A temperature output, as sensed bytemperature sensor 20, is supplied as a readout address to ROM 21 whichstores a table of reproducing laser power setting reference values REF.The reproducing laser power setting reference values, which differ withtemperature outputs from temperature sensor 20, are read from ROM 21.The read-out setting reference values REF are latched by latch circuit22 by timing signals from timing signal generator 23 before beingsupplied to automatic power controlling circuit 16, as a result of whichthe output power of the laser light source 12 is controlled so as toconform to the setting reference value REF conforming in turn to theprevailing temperature of the optical disc 100.

With the variable reflectance optical disc 100, such as a phase changetype disc, similarly to the above-mentioned magneto-optical disc,temperature distribution by the laser beam scanning spot is shifted withchanges in the disc temperature, however, if the laser power output ischanged, the size of the region having the reflectance changed ischanged, even although the temperature of the optical disc 11 remainsconstant. Therefore, the size of the reproducing region may bemaintained constant by controlling the laser power depending ontemperatures even although the temperature of the optical disc 100 ischanged.

ROM 21 stores a table for laser power setting reference values REFcorresponding in a one-for-one relationship to minute temperatureranges, obtained by subdividing the temperature range of themagneto-optical disc 11 in accordance with the storage capacity of ROM21. The reproducing laser power setting reference values REF associatedwith temperature ranges of the optical disc 100 which will give constantoptimum sizes of the reproducing region are previously detected and thereproducing laser power setting reference values REF associated withthese temperature ranges are written in ROM 21. Whether or not the sizeof the reproducing region is of a optimum constant size may be detecteddepending on whether or not the RF signal level from signal processor 15is of the predetermined value when the information of the predeterminedreference pattern is reproduced.

Thus the reproducing laser power setting reference values REF conformingto the temperatures of the optical disc 100 as sensed by temperaturesensor 20 are perpetually read from ROM 21. The reproducing laser powersetting reference values REF need not be changed continually. Thus anoutput from ROM 21 is latched by latch circuit 22 by a timing signalfrom timing signal generator 23 and he reproducing laser power settingreference value REF is changed at the time point of the timing signal.The setting reference values REF are changed e.. at the reproductionstart point and at intervals of predetermined time, such as ten minutes,during reproduction, at which temperature changes are thought to beproduced. Thus the timing signals are generated by timing signalgenerator 23 at the time of start of reproduction and at an intervaloften minutes since the reproduction start time.

In this manner, the size of the reproducing region may be maintainedconstant by controlling the laser power, even although the temperatureof the variable reflectance type optical disc 100 is fluctuated withchanges in the ambient temperature to assure stable reproduction at alltimes.

The embodiment of FIG. 9 may be modified in the same manner as whenusing the above-mentioned magneto-optical disc. For example, the discrotating driving system may be CAV(Constant Angular Velocity) orCLV(Constant Linea Velocity). Besides, the intensity of the of thereadout beam may be controlled based on the detected medium temperature,or the size of the portion where reflectance is changed may be detectedbased on the level of the signal read from the optical recording medium.The setting values may be found by processing instead of by using theROM, while the disc surface temperature may be detected y and IR sensorinstead of by detecting the ambient temperature of the disc.

As an example of the optical disc 100 of the variable reflectance typeemployed in the embodiment of FIG. 9, a phase change type disc isexplained, in which a phase change material layer which may becrystallized after melting is used and in which, when the layer isirradiated with the readout beam, the layer of the phase change materialis partially liquefied in a melt crystallization region in the readoutbeam spot to be changed in reflectance and is reverted to the crystalstate after readout is terminated.

Referring to a schematic cross-sectional view of FIG. 10, the phasechange type optical disc, used as the optical disc 100 shown in FIG. 9,a layer of a phase change material 104 is formed via a first dielectriclayer 103 on a transparent substrate 102 on which phase pits are formed(on the lower side in the drawing), a second dielectric layer 105 isformed on the layer 104. (on the lower side in the drawing, hereinafterthe same) and a reflective layer 106 is formed on the second dielectriclayer. Optical characteristics, such as reflectance, are set by thesefirst and second dielectric layers 103 and 105.

If necessary, a protective layer, not shown, may be additionallydeposited on the reflecting layer 106.

As alternative constitutions of the phase change type optical discs,only the phase change material 104 may be intimately deposited directlyon the transparent substrate 102 on which pits are formed, as shown inFIG. 11, or the first dielectric layer 103, a phase change materiallayer 104 and a second dielectric layer 105 may be sequentially formedon the transparent substrate 102 on which phase pits are formed, asshown in FIG. 12.

The transparent substrate 102 may be a substrate of synthetic resin,such as a glass substrate, polycarbonate or methacrylate. Alternatively,a photopolymer layer may be deposited on the substrate and phase pitsmay be formed by a stamper.

The phase change material may be such materials which undergoes partialphase changes within a scanning spot of the readout beam and is resetafter readout and the reflectance of which is changed with phasechanges. Examples of the material include calcogenites, such as Sb₂ Se₃,Sb₂ Te₃, that is chalcogen compounds, other calcogenites or unitarycalcogenites, that is calcogenitic materials, such as Se or The,calcogenites thereof, such as BiTe, BiSe, In-Se, In-Sb-The, In-SbSe,In-Se-Tl,Ge-The-Sb or Ge-The. If the phase change material phase 104 isconstituted by chalcogen or calcogenite, its characteristics, such asheat conductivity or specific heat, may be rendered desirable inproviding satisfactory temperature distribution by the readout beam.Besides, the melted state in the melt crystallized region as laterexplained may be established satisfactorily to generate ultra-highresolution with high S/N or C/N ratio.

The first dielectric layer 103 and the second dielectric layer 105 maybe formed of, for example, Si₃ N₄, SiO, SiO₂, AlN, Al₂ O₃, ZnS or MgF₂.The reflective layer 106 may be formed of Al, Cu, Ag or Au, admixed withminor amounts of additives, if desired.

As a concrete example of the phase change type optical disc, an opticaldisc having an arrangement shown in FIG. 10 is explained. With thisoptical disc, a layer of a material which may be crystallized on beingmelted is formed on a transparent substrate on which phase pits areformed. When a readout beam is radiated, the layer of the phase changematerial is partially liquefied in a melted and crystallized regionwithin the readout scanning beam spot and is reset after readout o acrystallized state.

A so-called glass 2P substrate was used as the transparent substrate102. Phase pits 101 formed on a major surface of the substrate 102 wereof track pitch of 1.6 μm, a pit depth of about 1200 Å and a pit width of0.5 μm. A first dielectric layer 103 of AlN was deposited on one majorsurface of the transparent substrate 102 having these pits 101, to athickness of 900 Å, and a layer of a phase change material 104 of Sb₂Se₃ was deposited on the layer 103 on the lower surface thereof in FIG.10, hereinafter the same). A second dielectric layer 105 of AlN wasdeposited thereon and an Al reflective layer 106 was deposited thereonto a thickness of 300 Å.

The following operation was performed on a portion of the optical discfree from recorded signals, that is a mirror-surface part thereof freefrom phase pits 101.

A laser beam of e.g. 780 nm was radiated to be focused on a point of theoptical disc which was then initialized by being allowed to coolgradually. The same point was then irradiated with a laser pulse with alaser power P set to 0<P≦10 mw. The pulse width was set to 260nsec≦t≦2.6 μsec. The result is that, if the reflectance is changedbetween two solid phase states before pulse irradiation and after pulseirradiation followed by cooling to room temperature, the layer ischanged from a crystal state to an amorphous state. if the reflectanceis not changed during this operation, but the amount of return light isonce changed during irradiation of the pulse beam, it is an indicationthat the film of the crystal state is once liquefied and againcrystallized. The region in the melted state which has once becomeliquid and which may be returned to the crystallized state with loweringof temperature is termed a melted and crystallized region.

FIG. 13 shows the phase states of the layer of the phase change material104 of Sb₂ Se₃ and values of a pulse width of the radiated laser pulseplotted on the abscissa and the laser beam power plotted on theordinate. In this figure, a hatched area R₁ below a curve a indicates aregion in which the layer of the phase change material 104 is notmelted, that is maintained in its initial state. In this figure, theregion above curve a becomes liquid, that is melted, on laser beam spotirradiation. A region between curves a and b is a melted andcrystallized region which is reset to a crystal state when cooled toabout the ambient temperature by elimination of the laser beam spot andthereby turned into a solid phase. Conversely, a hatched region R₃ abovecurve b is a melted amorphous region which is rendered amorphous whencooled and turned into a solid phase by elimination of the laser lightspot.

In the present embodiment, the reproducing laser power, optical discconstitution, material and the film thicknesses are selected so that, inthe course of cooling to ambient temperature from the heated statecaused by readout beam irradiation during reproduction, the time Δtwhich elapses since the heated state brought about by irradiation of thereadout beam during reproduction until cooling to ambient temperaturebecomes longer than the time necessary for crystallization, so that thestate of liquid phase in the melted and crystallized region R₂ in FIG.13 will be produced during reproduction.

In the present embodiment, the reflectance in the initial state, that isin the crystallized state, was 57%, whereas that in the melted state was16%. When reproduction was performed with the playback power of 9 mW andthe linear velocity of 3 m/sec, the ratio C/N was 25 dB.

FIG. 14 shows the results of measurement of the phase change states foranother example of the phase change type optical disc making use of Sb₂Te₃ as a phase change material 104, similarly to FIG. 13. In FIG. 14,the parts corresponding to those of FIG. 13 are indicated by the samereference numerals. In the present example, making use of Sb₂ Te₃, thereflectance in the crystallized state, that is initial state, wa 20%,while that in the melted state was 10%.

Meanwhile, with calcogenites or chalcogens, such as Sb₂ Se₃ or Sb₂ Te₃,the reflectance for the amorphous state is substantially equal to thatin the melted state. The optical disc employed in the present embodimentis reproduced with an ultra-high resolution by taking advantage oftemperature distribution within the scanning spot on the optical disc.

Referring to FIG. 15, explanation is given of the case in which a laserbeam is radiated on the phase change type optical disc.

In FIG. 15, the abscissa indicates a position of the beam spot relativeto the scanning direction X. A beam spot SP formed on the optical discon laser beam radiation has a light intensity distribution as indicatedby a broken line a. On the other hand, temperature distribution in thelayer of the phase change type material 104 is shifted slightly rearwardrelative to the beam scanning direction X, as indicated by a solid lineb, in association with the scanning speed of the beam spot SP. If thelaser light beam is scanned in the direction shown by arrow X, theoptical disc as a medium is gradually raised in temperature, from thedistal side relative to the scanning direction of the beam spot SP,until finally the temperature becomes higher than the melting point MPof the layer 104. In this stage, the layer 104 is in the melted state,from its initial crystal state, and is lowered in e.g. reflectance, as aresult of transition to the melted state. The reflectance of a hatchedregion P_(x) within the beam spot SP is lowered. That is, the regionP_(x) in which the phase pit 101 can hardly be read and a region P_(z)remaining in the crystallized state exist within the beam spot SP. Thatis, even when two phase pits 101, for example, exist in one and the samespot SP as shown, it is only the phase pit 101 present in the highreflectance region P_(z) that can b read, whereas the other phase pit101 is present in the region P_(x) with extremely low reflectance andhence cannot be read. In this manner, only the single phase pit 101 canbe read even although plural phase pits 101 exist in the same spot SP.

Therefore, if the wavelength of the readout light beam is λ and thenumerical aperture of the objective lens is NA, readout can evidently bemade satisfactorily even with the shortest phase pit interval of therecording signals along the scanning direction of the readout light beamis not more than λ/2NA, so hat signals can be read with ultra-highresolution to increase the recording density, above all, the linedensity, and hence the recording capacity of the recording medium.

In the above embodiment, operating conditions, such as film thicknesses,are set so that the reflectance is low or high when the layer of thephase change material 104 is in the melted state or in the crystallizedstate, respectively. However, the thickness or the constitution of eachlayer or the phase change material may be so set that the reflectancebecomes high or low in the melted state or in the crystallized state,respectively, in which case a phase pit may be present in the hightemperature region P_(x) in the laser beam spot SP shown in FIG. 15 sothat only this phase pit in the high temperature region P_(x) is read.In the case of an irreversible phase change in which a region is rasedin temperature by laser beam irradiation to reach the melted andcrystallized region R₃ such that it cannot be reset to the initializedstate or crystallized state even if cooled to ambient temperature, it isonly necessary to perform some initializing operation without departingfrom the scope of the present invention. For example, by radiating anelliptical spot after the reproducing laser spot for heating the layer104 to the melted and crystallized region R₂, or by heating to atemperature lower than the melting point MP and nor lower than thecrystallization temperature, the layer 104 may be initialized by beingreset from the amorphous state to the crystallized state.

Although the reflectance is changed in the above embodiment by phasechanges of the recording medium, the reflectance may be changed bytaking advantage of any other phenomenon. Thus, for example, thereflectance may be changed by temperature by taking advantage of changesin spectral characteristics caused by moisture adsorption by aninterference filter according to a modified embodiment shown in FIG. 16.

In the embodiment shown in FIG. 16, materials with markedly differentrefractive indices are repeatedly deposited on a transparent substrate132, on which phase pits 131 are formed, to thicknesses equal to onefourths if the wavelength λ of the reproducing beam, for forming aninterference filter. in the present embodiment, an MgF layer 133 (with arefractive index of 1.38) and an ZnS layer 134 (with a refractive indexof 2.35) are used as the materials with markedly different refractiveindices. However, any other combinations of the materials having largerdifferences in refractive indices may be employed. For example, SiOhaving a lower refractive index of 1.5 may be used as a low refractiveindex material, and TiO₂ with a refractive index of 2.73 or CeO₂ withrefractive index of 2.35 may be used as a high refractive indexmaterial.

The above-mentioned MgF layer 133 or the ZnS layer 134 are deposited byevaporation. If the reached vacuum is set to a value of e.g. 10₋₄ Torrwhich is lower than a usual value, the film structure becomes porous topermit the moisture to be captured. With the interference filter formedby a film which thus has captured the moisture, the reflectance andspectral characteristics are changed markedly between the state in whichthe filter is at room temperature and the state in which the filter isheated to close to the boiling point of water. That is, an acutewavelength shift is observed, in which the spectral characteristics atroom temperature are as shown by a curve i having a point of inflectionat a wavelength λ_(R) while the characteristics at approximately theboiling point are as shown by a curve ii having a point of inflection atwavelength λ_(H) and returned to the characteristics shown by curve i onlowering the temperature. This phenomenon may be probably caused byacute changes in refractive index due to vaporization of the moistureresulting in changes in spectral characteristics.

Therefore, if the wavelength of the light source of the reproducing beamis selected to a wavelength λ_(O) intermediate between these points ofinflection λ_(R) and λ_(H), the reflectance is dynamically changedbetween the state of room temperature and the heated state.

In the present embodiment, high density reproduction is performed bytaking advantage of these changes in reflectance. The mechanism of highdensity reproduction is described in connection with FIG. 15. In thiscase, the region in which the moisture is vaporized to producewavelength shift corresponds to the high reflectance region, while theportion of the medium in which the temperature is not raised is the maskregion. In the present embodiment, since the reflectance characteristicsare reverted to the original state when the temperature is lowered, sothat no particular erasure operation is required.

By using the reflectance change type optical disc as the optical disc100 shown in FIG. 9, the size of the effective reproduction region (thehigher reflectance region of the regions P_(x) and P_(z) in FIG. 15) maybe rendered constant despite temperature changes of the optical disc100, so that reproduction may be performed stably to assure high qualityreproduction signals.

It is to be noted that the present invention is not limited to theabove-described embodiments, but may be applied to, for example, a card-or sheet-shaped optical recording medium besides the disc-shapedrecording medium.

What is claimed is:
 1. A method for reproducing an optical recordingmedium comprising a recording layer and a reproducing layer, saidrecording and reproducing layers being magnetically coupled to eachother in steady state, said method comprising rendering recorded dataunreadable by extinguishing magentic coupling between the recordinglayer and the reproducing layer in a region the temperature of which israised to a temperature higher than a predetermined temperature byirradiation of a readout laser beam during reproduction, and reading therecording information held by said recording layer in an area of anirradiated region other than the magnetic coupling extinguished region,characterized bydetecting the temperature of a irradiated portion ofsaid optical recording medium, and controlling the size of said magneticcoupling extinguished region based on the detected temperature.
 2. Themethod as defined in claim 1 further comprising controlling theintensity of said readout beam based on the detected temperature of saidrecording medium.
 3. The method as defined in claim 1 further comprisingcontrolling the size of the magnetic coupling extinguished region basedon the level of the signal read out from said reproducing layer.
 4. Amethod for reproducing an optical recording medium comprising arecording layer and a reproducing layer, said method comprising thesteps of aligning domains of said reproducing layer, transcribing therecording information held by said recording layer in a region of therecording medium in which the temperature is raised to a temperaturehigher than a predetermined temperature by irradiation of a readout beamduring reproduction for relieving the transcribed recording informationby rendering said recording information readable, within a relievedregion, and reading out the recording information from said relievedregion of said reproducing layer, characterized by detecting thetemperature of an irradiated portion of said optical recording medium,andcontrolling the size of said relieved region based on the detectedtemperature.
 5. The method as defined in claim 4 further comprisingimpressing a reproducing magnetic field when transcribing and relievingthe recording information held by said recording layer duringreproduction to said reproducing layer, and controlling the intensity ofthe reproducing magnetic field based on the detected temperature of therecording medium.
 6. The method as defined in claim 4 further comprisingcontrolling the size of said relieved region based on the level of asignal read from said reproducing layer.
 7. A method for reproducing anoptical recording medium having phase pits formed thereon in accordancewith signals and having its reflectance changed with temperature, saidmethod comprising radiating a readout beam to the recording medium, andreading out said phase pits while partially changing the reflectancewithin a scanning spot of a readout beam, characterized bydetecting thetemperature of an irradiated portion of said optical recording medium,and controlling the size of a portion within the scanning spot of thereadout beam in which reflectance is changed depending on the detectedtemperature.
 8. The method as defined in claim 7 further comprisingcontrolling the intensity of said readout beam based on the detectedtemperature of said recording medium.
 9. The method as defined in claim7 further comprising controlling the size of said portion in whichreflectance is changed depending on the level of the signal read outfrom said optical recording medium.